An external hallucination is the perception of a visual hallucination which displays itself seamlessly into the external environment as if it were actually happening.   This is in stark contrast to internal hallucinations, such as dreams, that occur exclusively within an imagined environment and can typically only be viewed with closed eyes. This effect is capable of manifesting itself across the 4 different levels of intensity described below:
- Fleeting hallucinations - At the lowest level, external hallucinations generally consist of movement within the peripheral vision and ill-defined, fleeting hallucinations which disappear once a person double takes.
- Vaguely defined hallucinations - At this level, the hallucinations are visible within one’s direct line of sight, but are not fully defined in their appearance. This means that, although visible, they do not look completely detailed and are often extremely blurry or semi-translucent with little, if any, colour.
- Partially defined hallucinations - At this level, the hallucinations become distinct enough in their detail and vividness to extend beyond transparent, colourless, or blurry manifestations. However, they still remain unconvincing and do not quite live up to the detail of everyday life.
- Fully defined hallucinations - At this level, the hallucinations have become completely realistic and will rarely disappear simply because a person double takes. They are now capable of a completely convincing and photorealistic appearance and their behaviour becomes far more lifelike. Additionally, they may become numerous enough to fully engulf the entirety of the person’s environment
The specific differences between each potential style of external hallucination can be broken down into the following variations:
- Lucid vs. Delirious - A person experiencing a hallucinatory state can maintain a consistent level of awareness regarding the fact that none of these events are actually occurring and that the effects are simply the result of a drug-induced hallucination. In contrast with this, hallucinations may also become completely believable, no matter how nonsensical they may be, in exactly the same way that a person does not have any problem accepting absurd and non-linear plots within our dreams.
- Interactive vs. Fixed - Hallucinatory states can either present themselves as completely separate in a manner that is similar to watching a video play out in front of one’s field of vision or they can be completely interactive. For example, it is entirely possible to converse with autonomous entities or interact with imagined objects in a fashion similar to lucid dreaming.
- New vs. Old experiences - In terms of their subject matter, hallucinations can either be entirely new experiences or they can be old, everyday experiences and replayed memories.
- Controllable vs. Autonomous - Imagery and hallucinations can be partially to completely controllable. This can be described as their content always seeming to perfectly follow and fit the general topic and subject matter of one’s current thought stream, with varying levels of control. In contrast, autonomous hallucinations are completely spontaneous in their subject matter and entirely uncontrollable.
- Geometry-based vs. Solid - Hallucinations can be comprised of condensed psychedelic geometry or they can appear to be made from realistic materials depending on the substance consumed.
-  Gauntlett-Gilbert, J., & Kuipers, E. (2003). Phenomenology of visual hallucinations in psychiatric conditions. The Journal of nervous and mental disease, 191(3), 203-205. | https://doi.org/10.1097/01.NMD.0000055084.01402.02
- Obreshkova, D., Kandilarov, I., Angelova, V. T., Iliev, Y., Atanasov, P., & Fotev, P. S. (2017). Pharmaco-toxicological aspects and analysis of phenylalkylamine and indolylalkylamine hallucinogens. Pharmacia, 64(1), 32-47. | http://bsphs.org/wp-content/uploads/2017/04/Angelova.pdf